Security for foreigners in Saudi Arabia continued to deteriorate yesterday as the authorities in the kingdom struggled to locate an American man taken hostage over the weekend by a group claiming to be linked to al-Qa'ida.

Security for foreigners in Saudi Arabia continued to deteriorate yesterday as the authorities in the kingdom struggled to locate an American man taken hostage over the weekend by a group claiming to be linked to al-Qa'ida.

Security officials and diplomats said a body, possibly of a Westerner, was found in Riyadh, although the capital's police chief later denied this.

It appeared that the abducted man was Paul Johnson, an engineer with the American defence company Lockheed Martin. Investigators were struggling to identify the dead man, but sources said they were working on the assumption that it was not Mr Johnson.

Details of Mr Johnson's apparent abduction were carried in a statement on an Islamist website showing his picture on a Lockheed Martin identity card. The same statement claimed responsibility for gunning down another US defence contractor, Robert Scroggs, as he drove into his Riyadh home on Saturday. This brought to three the number of Westerners killed in Saudi Arabia in a week.

The apparent abduction of Mr Johnson and the killing of Mr Scroggs mark the latest escalation of a campaign to destabilise the kingdom. There are about 8.8 million foreigners in Saudi Arabia working in areas, such as the oil and financial sectors. A week ago, Simon Cumbers, an Irish cameraman working for the BBC, was killed in Riyadh and, in the same attack, Frank Gardner, its security correspondent, was critically wounded.

The Foreign Office said yesterday that it had authorised non-essential staff at the British embassy in Saudi Arabia and their relatives to leave the country if they wish. British Airways said its crews would no longer stay overnight in Saudi Arabia. Staff on flights from Heathrow will now leave the plane at Kuwait, while another BA crew takes the aircraft on to Saudi. That crew will remain in the airport while the plane is refuelled, and will not stay in the country overnight.

Colin Powell, the US Secretary of State, said yesterday that Saudi Arabia was mobilising its forces to quell the attacks. "The Saudis know that this is an enemy that is coming after them. The killing of foreigners ... is a direct attack against the Saudi regime. It's trying to disrupt normal commerce, disrupt the oil sector," General Powell said on ABC.

Condoleezza Rice, the US National Security Adviser, said on CNN that the US had repeatedly warned Americans to leave Saudi Arabia. She added that "we are doing everything we can with the Saudis to protect those who are there but it is obviously a dangerous place".

The statements from the terrorists make it clear that they are acting out of revenge for the abuse of Iraqi prisoners in Abu Ghraib prison. Mr Johnson was singled out because he worked on Apache helicopters. "These ... are used by the Americans, their Zionist allies and the apostates to kill Muslims, terrorising them and displacing them in Palestine, Afghanistan and Iraq," the statement said.

The group identified itself as "al-Qa'ida in the Arabian Peninsula", the same group that carried out the mass hostage taking in the city of Khobar last month that left 22 people dead.